$basetexture is the first texture
$basetexture2 is the second texture
%tooltexture is what texture will show up in the hammer material viewer thing (you could make your own custom one)

%detailtype is used for random placement of sprites and the such, which I shall explain in a tutorial later on today!

The rest you just leave as default.

Now, place the texture in the correct places;

Firstly, in you materials dir for Hammer (mine is d:\steam\steamapps\my@address.com\sourcesdk\sdkcontent\hl2mp_sample_content\materials\) and in your HL2 mod dir (D:\Steam\SteamApps\my@address.comk\half-life 2 deathmatch\hl2mp\materials\).
Of course, these depend on what game you are mapping for, but you should get the idea.

Make a displacement brush with your new texture, and paint some alpha. With any luck you will get something like: